Warning: This plugin allows login CSRF by design. You are likely better off using a proper single-sign-on solution such as the Nextcloud SAML application.
Login users to your Nextcloud with a simple HTTP POST request from another web page, the following form explains the usage of this plugin:
<html>
<body>
<form method="post" action="https://example.com/index.php/apps/loginviapost/login">
<input type="text" name="username" />
<input type="text" name="password" />
<input type="submit" value="Submit" />
</form>
</body>
</html>